The structure and function of a muscle are intimately related. Information about the cellular and sub-cellular levels of organization can be used to estimate the force, speed of contraction, speed of relaxation, and other parameters of muscular performance. Similarly, data that describe a muscle's performance can be used as indicators of its morphology. Although the value of these concepts as tools to elucidate the details of muscle mechanics has been generally recognized, they have been applied almost exclusively to one narrow class of muscles, namely vertebrate striated muscle types, particularly invertebrate smooth muscle, has been virtually untested. This project's aim, therefore, is to quantitatively describe the structure and function of invertebrate smooth muscle and test the generality of striated muscle performance parameters. This information will then be applied to address the underlying assumptions of muscle mechanics. The model system chosen for this study is the gastropod pedal muscle, which provides a diversity of morphologies and contractilities. The flexible, fleshy foot of a gastropod is a solid muscle mass in which bundles of muscle fibers are interwoven into an integrated whole. While the morphology of the individual muscle fibers resembles vertebrate smooth muscle, the connections between fibers and the mechanics of their contractions more closely resemble vertebrate cardiac muscle. Using an integrative functional morphological approach, a hierarchy of morphological observations and measurements ranging from changes in shape and volume of moving feet to dimensions of thick and thin filaments will be combined with an analysis of each muscle's performance. This information will be used to test whether the morphological correlates of function traditionally applied to vertebrate striated muscle are indeed generalizable to other muscle types. In addition, it will serve as a foundation for comparative studies of invertebrate and vertebrate muscle systems.